If it is possible to remove the dressing when doing ghusl and wash the skin beneath it, then put it back without it causing any harm, then this is what must be done and it is not permissible to wipe over it. 

But if harm will be caused by removing the dressing, then there is nothing wrong with wiping over it. 

Ibn Qudaamah (may Allaah have mercy on him said): Ahmad said: When he does wudoo’, if he fears that the water may harm his wound, he may wipe over the dressing. 

Similarly if he has put medicine on his wound, and he is afraid to take it off, he may wipe over it. This was stated by Ahmad. Al-Athram said: I asked Abu ‘Abd-Allaah about a wound on the leg on which medicine is put, and one is afraid that removing the medicine when doing wudoo’ may cause harm. He said: I do not know what will harm him, but if he is afraid for himself or is afraid about that, then he may wipe over it. 

Al-Qaadi said concerning dressings on wounds: If taking it off will not cause harm then he should take it off, but if taking it off will cause harm, then the ruling is the same as that on splints, and he may wipe over it. End quote from al-Mughni (1/172). 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: What is the ruling on leading the people in prayer in congregation when one has a wound? 

He replied: If the wound is covered with a splint, he may wipe over it when doing wudoo’ or ghusl in the case of janaabah, and that is valid, and his prayer is valid, whether he is leading the prayer, praying behind an imam or praying alone. If there is no splint on it, he may do tayammum for it after washing the parts of his body that are sound, and that is valid and his prayer is valid, because Allaah says (interpretation of the meaning): “So keep your duty to Allaah and fear Him as much as you can” [al-Taghaabun 64:16]. And the Prophet (peace and blessings of Allaah be upon him) and the Muslims who were wounded on the day of Uhud prayed with their wounds. And Abu Dawood (may Allaah have mercy on him) narrated from Jaabir (may Allaah be pleased with him) that a man was wounded and some of his companions advised him to do ghusl, so he did ghusl and died. The Prophet (peace and blessings of Allaah be upon him) said: “They killed him, may Allaah kill them. Why didn’t they ask if they did not know? The remedy for not knowing is asking.” Then the Prophet (peace and blessings of Allaah be upon him) said: “It would have been sufficient for him to put a dressing on his wound and wipe over it, and wash the rest of his body.” End quote from Fataawa al-Shaykh Ibn Baaz (10/119). 

We ask Allaah to heal you. 

And Allaah knows best.